This invention pertains to a digital FM demodulation circuit in general. More specifically, the invention pertains to a circuit for the digital FM demodulation of temporarily equidistant samples derived from an analog frequency-modulated signal by way of sampling with the aid of a sampling signal. The samples are fed to the input of a chain of two series-arranged delay stages each having a delay equal to the period of the sampling signal. The samples are coupled to the first input of an adder whose second input is coupled to the output of the last delay stage. The output of the adder is connected to the dividend input of a divider which has its divisor input coupled to the output of the first delay stage. The divider output signal is used either directly or after forming the corresponding inverse sign values to provide a demodulated digital signal.
One such circuit is known from the German Offenlegungsschrift No. DE 30 30 853 A1. In this publication it is stated as a general principle that the digital FM demodulation can be achieved by a corresponding interconnection of three samples. However, more specifically the digital FM demodulation is obtained from three successively following samples as follows: the first and the third sample are added and divided by the second sample. Implementing this principle in circuitry leads to the circuit described above.
As is well known from the sampling theorem, the sampling frequency must be at least twice as high as the highest frequency existing in the signal to be sampled. To demonstrate a disadvantage of the conventional circuit, reference is first made to the special case in which the frequency of the sampling signal is just four times as high as the frequency of the signal to be sampled, and the signal frequency is frequency-modulated in the usual way. In that case, the sign of the first sample is opposite to that of the associated third sample of three successively following (adjacent) samples. In addition the amounts of both the first and the third sample are practically identical. This implies that the sum formed from these two samples results in small a numerical value lying around the zero point which, to provide sufficient resolution, requires a correspondingly high number of digits in the associated digital signals and therefore in the corresponding digital words. This property of the conventional circuit is of importance especially in cases where the first and the third sample are obtained adjacent to the positive and the negative peak value of the signal to be sampled.